The present invention relates to the detection of trace gas species including explosives, with high sensitivity and specificity, and more particularly to tunable laser-based systems for trace detection of nitrogen dioxide.
The real-time monitoring of trace gas species is of great interest in a wide range of fields, including environmental science, safety monitoring and air quality control (e.g., for compliance with Environmental Protection Agency regulations), defense and homeland security (e.g., for the detection of trace amounts of explosives or explosive compounds), and non-invasive medical diagnostics (e.g., breath analysis). See for example, United States Environmental Protection Agency, “National Air Quality Status and Trends Through 2007, EPA-454/R-08-006”, United States Environmental Protection Agency, Washington, D.C. (2008); J. Hildenbrand et al., “Explosive detection using infrared laser spectroscopy,” Proc. SPIE 7222, 72220B-1-12 (2009); T. H. Risby, S. F. Solga, “Current status of clinical breath analysis,” Appl. Phys. B 85, 421-426 (2006) and a presentation by the present inventor, Gottipaty N Rao and by A. Karpf, “Sensors at ppb Sensitivity or Better Based on Multiple Line Integration Spectroscopy Techniques,” Laser Applications to Chemical, Security and Environmental Analysis (LACSEA) 2010 paper: LPDP2.
In these applications, the concentrations of the trace gas species typically range from parts per trillion (10−12) to parts-per-million (10−6) levels and thus require techniques that are both highly sensitive and selective. Nitrogen oxides (NOx) are of particular interest since they are some of the most damaging of these pollutants. They play important roles in the formation of photochemical smog, the formation of tropospheric ozone, and the formation of acid rain, and thus can directly impact public health.
Laser-based techniques for trace gas detection have many advantages over other techniques because of their ability to provide real-time monitoring capabilities with greater sensitivity and selectivity. In particular, Quantum Cascade Lasers are attractive sources for trace gas detection because they operate in the mid-infrared region (λ=4 μm to 24 μm) and thus provide access to the fundamental rotational-vibrational transitions of molecular species. Examples may be found in A. A. Kosterev et al., “Chemical Sensing with pulsed QC-DFB lasers operating at 6.6 μm” Appl. Phys. B. 75, 351-357 (2002); F. K. Tittel et al., “Recent Advances in Trace Gas Detection Using Quantum and Interband Cascade Lasers,” Rev. of Laser Eng. 34, 275-282 (2006); A. Kosterev et al., “Application of quantum cascade lasers to trace gas analysis,” Appl. Phys. B 90, 165-176 (2008); R. F. Curl et. al., “Quantum cascade lasers in chemical physics,” Chem. Phys. Lett. 487, 1-18 (2010); and a recent review article by the inventor, Gottipaty N Rao and by A. Karpf, “External cavity tunable quantum cascade lasers and their applications to trace gas monitoring,” Applied Optics, Vol. 50 Issue 4, pp. A100-A115. (2011).
The quantum cascade lasers offer improved sensitivity of several orders of magnitude over near-infrared diode laser based techniques that employ the detection of the overtones of the molecular transitions. Quantum cascade lasers have been used to detect several trace gasses (e.g., CO, CO2, NO, NO2, NH3, CH4 and N2O) with different spectroscopic techniques (e.g., Laser Absorption Spectroscopy, Cavity Ringdown Spectroscopy, Integrated Cavity Output Spectroscopy) as described in the review article by the present inventor Gottipaty N Rao and by Andreas Karpf. By employing an external cavity arrangement, a quantum cascade laser provides a narrow line width (Δν˜0.001 cm−1), highly stable and reproducible tunable continuous wave (“CW”) output, and a wide continuous tuning range; all of which are essential for the study of complex spectra. The investigation of trace gas species involves just such a study of complex spectra. See G. Wysocki, et al., “Widely tunable mode-hop free external cavity quantum cascade laser for high resolution spectroscopic applications,” Appl. Phys. B 81, 769-777 (2005) and articles by the present inventor, A. Karpf, G. N. Rao, “Absorption and wavelength modulation spectroscopy of NO2 using a tunable, external cavity continuous wave quantum cascade laser,” Appl. Opt. 48, 408-413 (2009); Gottipaty N Rao et al., “A Trace Gas Sensor at ppb Sensitivity Based on Multiple Line Integration Spectroscopy,” Conference on Lasers and Electro-Optics (CLEO) 2010 paper: JWA60.
Laser absorption spectroscopy is based on recording the change in intensity of laser radiation as it passes through a region containing the sample of interest. As the laser is tuned across a transition, the transmitted laser intensity is a function of frequency ν given by Beer's law:I(ν)=I0(ν)e−α(ν)L  (1)where I0 is the incident laser intensity, L is the optical path length, α(ν) is the absorption coefficient at frequency ν. In the low concentration regime (where α(ν)L≦0.05) one can approximate equation (1) as:I(ν)=I0(ν)[1−α(ν)L].  (2)
When using laser absorption spectroscopy, the sensitivity of a spectrometer is often determined by taking the ratio of the amplitudes of the absorption line to that of the noise level.
The sensitivity of a laser based sensor may be enhanced by using frequency modulation techniques. As the laser is slowly scanned across a transition, it is also modulated at a higher frequency (f) and detection is done at higher harmonics of the frequency (e.g., 2f). This is done so that i) low frequency drifts in laser intensity do not affect the absorption measurements and ii) excess laser noise, which varies as 1/f, is reduced thereby enhancing the signal-to-noise ratio. These techniques are characterized as either Frequency Modulation (FM) or Wavelength Modulation Spectroscopy (WMS), based on the frequency at which the laser is modulated. Specifically, FM spectroscopy is when the laser is modulated at frequencies greater than the absorption line-width, while WMS is when the modulation frequencies are lower than the absorption line-width. See, J. Reid and D Labrie, “Second-Harmonic Detection with Tunable Diode Lasers—Comparison of Experiment and Theory,” Appl. Phys. B 26, 203-210 (1981) and an article by the present inventor, G. N. Rao along with C. Gudipaty and D. Martin, “Higher Harmonic Detection Employing Wavelength Modulation Spectroscopy and Near Infra-Red Diode Lasers: An Undergraduate Experiment,” Am. J. Phys. 77, 821-825 (2009).